The present invention relates generally to reinforcements for industrial belts and the like, such as conveyor belts, power transmission belts, etc., and more particularly to a fabric which may be embedded in the elastomeric material of the belt to increase the tensile strength of the latter so that the belt can perform satisfactorily as a conveying or power transmitting medium.
It is well known to embed a fabric in an elastomeric material such as rubber (natural or synthetic) to reinforce the latter. Cotton and other textiles of staple fiber material have been used heretofore with moderate success in that to some degree the rubber material is reinforced thereby. However, reinforcements made of these textiles display poor resistance to shocks, impact and severe blows to which rubberized belts incorporating same are subjected in their normal course of use.
Synthetic textiles such as that constituted of polyester have therefore been used of late with increasing frequency to overcome the disadvantages associated with the cotton-like textiles aforementioned. The polyester material utilized generally is that of the "spun" variety or that of the "continuous filament" variety.
Conventional polyester yarn for reinforcing fabrics, whether the yarn is spun yarn or filament yarn, is generally a tightly woven yarn having a plurality of single ends plied together. Each single end is usually first subjected to a ring spinning process and then the single ends are together twisted about one another so as to ply them into a single cord. With regard to spun yarns, each yarn cord is defined in terms of a conventional designation of "cotton count" and by the number of single ends which are plied to one another.
Historically, a numerical designation in the form of a fraction is used to define both the cotton count and the number of single ends plied to one another. In this respect, the numerator of the fraction represents the cotton count, namely a number by which a constant (not shown in the numerator) such as 840 is multiplied. The numerator has units in the form of yarn length per unit of weight. The units associated with the constant 840 are yards per pound. The denominator of the fraction designates the number of single ends plied to one another to form a single cord. The fraction when reduced to lowest terms gives rise to still another characteristic of the yarn, namely an "equivalent cotton count". When two different spun yarns are compared to one another with regard to properties in terms of bulk, tensile strength, etc., the yarns are generally compared on the basis of having substantially identical equivalent cotton counts.
It has been determined heretofore that it is usually good practice to initially subject the fabric to a conventional RFL (resorcinol formaldehyde latex) dip, the dip not only somewhat increasing the tensile strength of the yarn material but likewise increasing the adhesion of the yarn material to that of the rubber or rubber-like material in which the fabric is to be embedded to form, for example, a reinforced belt.
Conventional practice with spun yarns has been to rely on the mechanical adhesion of the bulky and fuzzy fibers thereof to the rubber or rubber-like material rather than merely a chemical adhesive bond therebetween. Thus, the degree of bulk of a particular spun yarn has a direct bearing on its capacity to mechanically adhere to the rubber or rubber-like material of the belt in which it is embedded. The greater its bulk, the greater is its adhesion to rubber. Accordingly, a highly bulky yarn which has been dipped in an RFL bath displays extremely desirable mechanical and chemical adhesive properties and lends itself for use with good success as a belt-reinforcement means.
A disadvantage associated with conventional reinforcing fabrics constituted of spun yarn is that the yarn is of the type having many plied ends, for example a 7/7 or 8/8 yarn. This type of yarn does not have substantial bulk, despite its spun nature, because the ends are tightly twisted and plied to one another. As a result, such yarn does not present optimum adhesion to rubber.
Moreover, such multiple plied spun yarn when weaved into a fabric is quite thin. As a result, when a belt employing such a fabric is joined at its opposite ends to present a closed loop (for example transmission belts) the clamps or rivets used for joining the belt-ends can be loosened and pulled out of the latter when the belt is subjected to tension below the tensile strength of the belt itself. This is so because as much as 80% of the effective clamping capacity of such clamps or rivets depends directly on the degree to which the fabric is squeezed by the clamp. Only 15% of the effective clamping capacity of these clamps depends upon the weft cords or picks of the fabric, and only 5% thereof is due to the rubber or rubber-like material in which the fabric is embedded.
Conventional practice, therefore, is to increase the number of superposed layers of such fabric in the rubber of the belt to thereby increase the effective fabric thickness which the clamp can "bite". Thus, the breaking strength of the clamped ends of the belt (or "weak link") is substantially increased by increasing the number of fabric layers, but at the additional cost of an excessive number of fabric layers.
This disadvantage associated with the degree to which the opposite belt ends can be effectively clamped when reinforced by a low bulk (and therefore thin) spun yarn fabric, is also associated with belts reinforced by fabrics of the continuous filament variety. As is well known, filament fabrics are likewise quite thin and therefore, in the absence of a multiplicity of superposed fabrics embedded in the rubber material of the belt, such filament fabrics likewise do not present a sufficient bite thickness for the clamps.
A further disadvantage associated with the many plied spun yarn in particular is that experience has determined that after repeated use, for example several months, of a belt in which is disposed such a reinforcing yarn, the belt undergoes a reduction in tensile strength. As a result, such a reinforcing yarn is not reliable over extended periods. The latter disadvantage is believed to be a result of the number of single ends which are plied to one another and, thereby, interlocked and restrained against permitting a uniform linear distribution therealong of repeated stresses. The interlocking of the single ends with one another is believed to form local high stress zones in the yarn which tend to weaken after extended use.